of, like or pertaining to the depths of the ocean; referring to or occurring in the region of deep water above the floor of the ocean.

Etymology: English abyss, from Middle English abissus, from Late Latin abyssus, “a bottomless gulf” + pelagic, from Latin pelagicus and possibly pelagus; from Ancient Greek pelagikós, from pélagos, “sea”.

[Pete Mohrbacher - Rahab, Angel of the Deep]